Growing concerns over the environmental impacts of non-biodegradable plastic waste and the need for sustainability have stimulated research efforts on biodegradable polymers from renewable resources. Rising costs and dwindling petrochemical feedstocks also make renewable resource-based materials attractive alternatives to their petroleum-based counterparts. Many of these efforts have concerned ester containing polymers such as polyesters, polyester amides, and polyester urethanes, where the polar ester groups (—COO—) offer biodegradability through hydrolytic and/or enzymatic degradation, and hydrophobicity through the long aliphatic segments. Aliphatic polyesters have applications in biomedical applications, packaging applications, and in the coating industry. The polyesters are monomers in the production of reprocessable polymers like thermoplastic polyester urethanes (TPEU)s. Although currently produced from depleting petrochemical resources, ingredients sourced from renewable feedstocks, such as natural oils, are increasingly used to develop such materials. In particular, TPEU elastomers made with aliphatic polyester segments have been shown to be biodegradable.
The control of the molecular weight of the polyesters is important as it determines the crystalline structure, further affecting mechanical and thermal properties of lipid derived TPEUs for a large part. In the particular case of polyester diols (PEDs, Scheme 1), used for the synthesis of TPEU elastomers, molecular weights between 1000 and 5000 gmol−1 are common.
Since the degree of polymerization is a function of reaction time, a specific molecular weight can be obtained by cooling the reaction at a suitable time. However, the polyesters obtained in this way will possess end-groups that can further react with each other, affecting polymer molecular weight. This situation can be resolved by operating under stoichiometric imbalance conditions in which one of the two reacting monomers is kept in excess, resulting in polymers with the same end-groups, terminating the polycondensation. In the case of the synthesis of PEDs, the diol is kept in excess and the polyesterification reaches a point where the deficient monomer (the diacid) is entirely reacted and the chain ends of the formed polymer is alcohol terminated, preventing further polymerization.
The rate of formation of linear polyesters is complicated by competing effect of side reactions such as intra-molecular cyclization due to the reaction of a terminal hydroxyl group with an internal ester group and/or terminal carboxylic acid of the same chain. Also, breakdown of polymeric PEDs caused by the alcoholysis of polyester groups of one chain by the terminal hydroxyl group of another chain occurs. The kinetics of polymerization of PEDs synthesized from unequal diacid and diol concentrations provides an indication of the impact of stoichiometric imbalance on molecular weight growth.
Polyesterification is a step polymerization reaction in which molecular weight increases slowly and takes long times to complete. It also promotes unavoidable volatilization of the monomers or precipitation of the polymer segments and the formation of low molecular weight volatile or cyclic oligomers, impacting the molecular weight and polydispersity index (PDI) of the polyester. The effective removal of the undesirable products is relevant to improving final TPEU properties. Some of the impurities can be removed by volatilization under vacuum at high temperature during the reaction, by column chromatography or fractional precipitation after the reaction. The latter is the most effective way for homogenizing molecular weight distribution based on the discriminatory solvent solubility of polymer chains.

The present effort describes the synthesis, purification and polymerization kinetics of lipid based PEDs. Molecular weight and distribution controls were achieved by initial and induced stoichiometric imbalance and optimization of the purification protocol. Thermal degradation and thermal transition behavior of the PEDs were investigated by TGA and DSC, respectively.